1. Field of the Invention
This invention relates to a plug valve and to a method of making such valve.
2. Prior Art Statement
It is known in the art to provide a plug valve for controlling fluid flow therethrough, with such valve having an outer housing or body which has a flow passage extending through the housing and including an inlet and an outlet interconnected by a bore disposed therebetween. The bore has a central axis disposed substantially perpendicular to the flow passage and a plastic sleeve is disposed within the housing and lines the bore. The plastic sleeve has a pair of aligned ports therein at diametrically opposed locations with each port being defined by annular edge surface means. The ports are aligned with the inlet and outlet to enable fluid flow through the passage. In such a plug valve a rotatable plug is disposed within the sleeve and the plug has an opening therethrough which is adapted to be aligned in and out of registry with the inlet and outlet in accordance with rotation of the plug relative to the sleeve and housing. The plug valve also has means for holding the sleeve against rotation relative to the housing. In such a prior valve, the plastic sleeve provides the primary fluid seal for the valve. Further, the holding means generally consists of what will be referred to as body lips defined as an integral part of the valve body and such lips are associated with the inlet and outlet in the housing. The lips of a typical known valve shield the plastic sleeve liner from direct impingement of fluid flowing through the valve and protect it from any abrasive material entrained in the fluid. The body lips also prevent rotation of the sleeve liner relative to the valve body whenever the plug member is rotated between open and closed positions.
It is common practice in such a prior plug valve to use a sleeve liner of polytetrafluoroethylene or equivalent material and with such a sleeve liner there is a strong tendency for the liner to be drawn toward the center line of the valve when the rotatable plug portion is in a nearly closed position. When the plug member is in this nearly closed position, the velocity of the fluid proximal to the side of the port is markedly increased with a resultant decrease in pressure. This reduction in pressure tends to pull and dislocate the plastic sleeve liner. The body lips also serve to prevent inward deflection of the plastic sleeve liner under these flow conditions and such body lips also prevent cold flow or extrusion of parts of the sleeve liner into the flow passage.
In the past, these body lips have been made as an integral part of the valve outer housing or body, as by casting. However, the casting of these lips in the valve body increases the manufacturing cost of such valve body and makes machining of the valve body very expensive. In addition, the body lips also cause problems in the assembly of the plastic sleeve liner in the valve body and these problems add to the cost of the overall valve.
It is also common practice with prior art plug valves to replace the entire outer housing or body once the body lips become damaged or defective. However, once a plug valve is in field service, such replacement is often difficult. Moreover, the special tooling and manufacturing techniques required in the assembly of a sleeve type liner in a valve body having body lips make in-field replacement of the sleeve type liners very difficult.
In view of the above, it is clear that prior art plug valves have numerous deficiencies.